Elaboration and characterization of ferroelectric materials Sr2(1-x)Na(1+x)GdxNb5O15 synthesized by solid process

Abstract

The solid solutions Sr2(1-x)Na(1+x)GdxNb5O15 (SNGN) with substitution rates of Sr2+ by Gd3+ varying between 0 and 1 were prepared and have undergone several heat treatments. The XRD spectra show that the powders calcined at 650 °C for 6 h exhibit a mixed structure consisting of orthorhombic-quadratic phases. On the other hand, calcination at 1200 °C for 12 h ensures complete crystallization of the SNGN compounds in the TTB structure of quadratic symmetry. The SEM images show grains of regular and spherical shapes. The Raman spectra of the solid SNGN solution calcined at 1200 °C (x = 0; 0.2; 0.4 and 0.6), show two broad bands attributed to the transverse deformation modes of the O-Nb-O bond and of elongation modes of the Nb-O bond of octahedra (NbO6). IR spectroscopy highlights the existence of absorption bands corresponding to the elongation and deformation vibrations of some bonds. The temperature measurements of the dielectric permittivity εr of the ceramic (SNN; x = 0) show for all the frequencies used, the presence of a narrow peak during the heating and the cooling phases with a maximum located respectively at the Curie temperature Tc ∼ 235.07 °C and Tc ∼ 224.84 °C. The dielectric losses tan (δ) values are very weak in the ferroelectric phase (T < Tc), then an increase, which reaches 2.3 for the lowest frequency, occurs in the paraelectric phase. The maximum permittivity ε'rmax undergoes a decrease as the frequency increases; this diffusive behavior can be associated to the domains walls in the micro-domains of the ferroelectric phase.